What is the pigtail inside the flange called

The coiled or looped section of tubing inside a flange is called a pigtail or pigtail siphon.

A pigtail is a looped or spiral section of pipe or tubing installed in piping systems, typically upstream of pressure measurement instruments such as gauges or transmitters, to protect them from high temperature, pressure spikes, and vibration . Its curved design allows it to flex under thermal expansion or pressure changes without transmitting stress to the connected equipment, enhancing the reliability and service life of the instruments . In steam or gas systems, a pigtail siphon (also called a lyre-type siphon) serves as a condensate trap. It allows vapor to condense into liquid before reaching the pressure gauge, creating a protective fluid barrier that prevents high-temperature steam or corrosive gases from damaging sensitive components . This design also dissipates heat and reduces vibration transfer, improving measurement accuracy and longevity . The pigtail is typically installed between the flange and the instrument, with the flange providing a secure connection point. It can be made from materials such as stainless steel or other corrosion-resistant metals, depending on the process media and operating conditions . Proper installation ensures that condensate collects in the coil, forming a protective layer that shields the instrument from direct exposure to the process fluid. In summary, the pigtail inside a flange is a protective, coiled section of tubing designed to buffer pressure, absorb vibration, and manage thermal effects, often referred to as a pigtail siphon or lyre-type siphon in instrumentation applications .